Solution:
Logic and Garageband uses a caching system to validate the Audio Unit plug-in for use in your project.
This process, unfortunately, can be temperamental, particularly following updates. This can lead to intermittent graphical glitches or the number of outputs being incorrectly recognized (EZdrummer may only appear as a stereo insert).
If you experience issues after an upgrade of either Logic or your Toontrack software please try in the first instance to delete the following files if they exist:
/Users/yourname/Library/Caches//AudioUnitCache/com.apple.audiounits.cache
/Users/yourname/Library/Caches/com.apple.audiounits.cache
/Users/yourname/Library/Caches/Logic/com.apple.LogicAudioUnits.plist
/Users/yourname/Library/Preferences/com.apple.audio.InfoHelper
Please know that the User library folder is hidden on Mac OSX 10.7 and above. To access the user library folder on 10.7 and above click (from the finder menu at the top of the screen) Go > Go To folder > Then type in: ~/Library
In recent versions of Logic, you can also try a Full Audio Unit Reset to rescan the Audio Units. Open the Logic Settings > Plug-in Manager and click the Full Audio Unit Reset button. Then restart Logic and it will start scanning.